Cryptosporidium parvum (or C. parvum) is an enteric protozoa of the phylum Apicomplexa. It is a major cause of diarrhea in humans and certain domestic animals (Tzipori, Advances in Parasitology (1988) 27:63–129). It is responsible for sporadic cases and major waterborne outbreaks of self-limiting diarrhea in immunocompetent humans (Current, W. L. et al., Clinical Microbiology Reviews, (1991) 4:325). C. parvum is one of several important opportunistic infections (OI) associated with diarrhea and wasting in patients with AIDS. Depending on location in the United States, some 10 to 15% of individuals with AIDS contract the disease (Peterson, Clinical Infectious Diseases, (1992) 15:903). The infection in the immunodeficient host often becomes persistent, causing life-threatening, profound, unremitting watery diarrhea and wasting. A prolonged course of infection often leads to a spread of infection into the hepatobiliary (HB) tract causing serious complications (Flanigan, Progress in Clinical Parasitiology (1993) 3:1). Of the OI affecting patients with AIDS, C. parvum is one of only a few infections against which there is no consistently effective treatment. There had been only a few reports of successful treatment of individual AIDS patients with hyperimmune bovine colostrum (Tzipori, Lancet. (1986) ii:344; Ungar, Gastroenterology (1990) 98:486) and with paromomycin (PRM) (Fitchenbaum, Clinical Infectious Diseases (1993) 16:298). Since none of the available antimicrobial agents are consistently effective, a search for novel therapeutic agents against C. parvum is necessary. With increased survival time of patients with AIDS due to much improved patient care, the incidence of the disease in this population is likely to continue to rise.
The lifecycle of C. parvum is similar to that of other coccidia which infect mammals. The lifecycle can be divided into six major developmental events (Current, Journal of Protozoology, (1986) 33:98); excystation, the release of infective sporozoites; merogony, the asexual multiplication within host cells; gametogony, the formation of micro and macrogametes; fertilization, the union of micro and macrogametes; oocyst wall formation, to produce an environmentally resistant stage that transmits infection from one host to another; and sporogony, the formation of infective sporozoites within the oocyst wall. Each intracellular stage of C. parvum resides within a parasitophorous vacuole confined to the microvillous region of the host cell, whereas comparable stages of Toxoplasma gondii, Eimeria, or Isopora to which C. parvum is closely related, occupy parasitophorous vacuoles deep within the host cytoplasm. Oocysts of C. parvum undergo sporogony while they are within the host cells and are infective when released in the feces. Approximately 20% of the oocysts of C. parvum are thin walled and discharge their sporozoites within the lumen of the same host, while 80% form a thick two-layered environmentally resistant oocyst wall, and are discharged in the feces. The four sporozoites emerging from the thin-walled oocysts and repeated cycles of schizogeny contribute to the persistence of the infection in the immunodeficient host known as autoinfection.